One type of semiconductor die, referred to as a "bumped" die, includes patterns of contact bumps formed on a face of the die. The contact bumps can be formed on wettable metal contacts on the die in electrical communication with the integrated circuits contained on the die. The contact bumps allow the die to be "flip chip" mounted to a substrate having corresponding solder wettable contacts. This mounting process was originally developed by IBM and is also known as the C4 joining process (Controlled Collapse Chip Connection).
Lead tin alloys (e.g., 95/5 lead tin alloy) and a ball limiting metallurgy (BLM) process can be used to form the bumps. Typically, the bumps are dome shaped, and have an average diameter of from 5 mils to 30 mils. Micro ball grid arrays (BGA) are formed in the smaller range, while standard ball grid arrays are formed in the larger size range. The sides of the bumps typically bow or curve outwardly from flat top surfaces. The flat top surfaces of the bumps form the actual regions of contact with the mating contacts on the substrate.
FIGS. 1A-1C illustrate a prior art flip chip mounting process. In FIG. 1A a bumped semiconductor die 10 includes a pattern of contact bumps 12 arranged in a desired pattern 14. As shown in FIG. 1B, the die 10 also includes a passivation layer 18 and contacts 16 for the bumps 12. The contacts 16 are in electrical communication with the semiconductor devices and integrated circuits formed on the die 10.
Each bump 12 can be formed on a corresponding contact 16. In addition, each bump 12 can include a stack of underlying layers 20a-c. By way of example, layer 20a can be an adherence layer (e.g., Cr), layer 20b can be a solderable layer (e.g., Cu) and layer 20c can be a flash layer (e.g., Au). The bumps 12 can be formed by processes that are known in the art such as ball limiting metallurgy (BLM). Typically, the bumps 12 comprise an alloy such as lead/tin or nickel/palladium.
In FIG. 1C the die 10 has been flip chip mounted to a substrate 22. The substrate 22 includes solder wettable contacts 24 embedded in a glass layer 26. During the flip chip mounting process the contact bumps 12 (FIG. 1B) on the die 10 are aligned and placed in physical contact with the contacts 24 on the substrate 22. This can be accomplished with an optical alignment device such as an aligner bonder tool. A flux can be placed on the substrate as a temporary adhesive to hold the die 10 in place on the substrate 22.
The temporary assembly is then subjected to a reflow thermal cycle using a heat source directed at the die 10 or an oven which heats the entire assembly. This melts the contact bumps 12 (FIG. 1B) and forms reflowed contact bumps 12RF. The reflowed contact bumps 12RF bond the contacts 24 on the substrate 22 to the contacts 16 on the die 10. In addition, the reflowed contact bumps 12RF provide separate electrical and heat conductive paths for the die 10.
In some applications an underfill layer 28 can be formed between the die 10 and the substrate 22. The underfill layer 28 seals the gap between the die 10 and substrate 22. In addition, the underfill layer 28 can include a heat conductive material, such as silver balls, to improve heat transfer from the die 10.
With flip chip mounting the physical attachment of the die 10 to the substrate 22 is formed by the reflowed contact bumps 12RF. In general, the reflowed contact bumps 12RF are relatively small in total area so that the attachment force is relatively low. In addition, the reflowed contact bumps 12RF can crack during subsequent usage of the substrate 22. This can loosen the die 10 and increase the electrical resistivity of the electrical paths between the die 10 and substrate 22.
Also during the flip chip mounting process, the die 10 must be held in place while the reflowed contact bumps 12RF harden from the molten state. Shifting of the die 10 during hardening of the reflowed contact bumps 12RF, can weaken the attachment forces between the die 10 and substrate 22. Still further, the die 10 must be pressed against the substrate 22 with a required pressure during the flip chip mounting pressure. This pressure also affects the subsequent attachment force. If the pressure is low or uneven the attachment force can also be low and uneven.
In view of the above limitations of conventional flip chip mounting processes, the present invention is directed to an improved system and method for attaching semiconductor dice to substrates.